Computational analysis of the influence of seat pan inclination and friction on muscle activity and spinal joint forces.

Abstract Objective To investigate how the combination of seat pan inclination and friction coefficient influences the internal forces in the seated human body. Background The focus on the ergonomics of sitting is increasing with the amount of time spent in a seated posture. A generic computer model of a chair has been developed and together with a detailed full-body musculoskeletal model it forms a new basis for investigation and understanding of the biomechanics of the seated human. Method The analysis based on inverse dynamics and a minimum fatigue criterion is used for solving the redundancy problem of the muscles. The input to the analysis is different combinations of seat pan inclination and friction coefficient; the output is the individual muscle activities and spinal joint forces. Results The computational investigations show that seat pan inclination and friction coefficient have a complex combined influence on muscle activity and spinal joint forces. Forward seat pan inclination appears to cause muscle fatigue unless sufficient friction is present, and in this case loses much of its beneficial effect for the spinal joint loads and leads to tissue shear forces that may be uncomfortable or even harmful. Conclusion A generic chair model has been developed; the model can be adjusted in numerous ways. The chair together with a full body musculoskeletal model forms an efficient tool, which can be used in the design process of new chairs for quantifying the consequences of various design choices. It seems difficult to combine the desire for relaxation with the desire for minimum spinal loads. Relevance to industry Detailed biomechanical modeling is a promising tool for furniture design, workplace design and indeed for improvement of occupational health.